So far, we’ve dealt with discrete pieces of data: one number, one
string, one value. When programming, it is more often the case that
you want to work with groups of data.
Clojure has great facilities for working with these groups, or
collections, of data. Not only does it provide four different types
of collections, but it also provides a uniform way to use all of these
collections together.
Vectors
Sequential collection
A vector is a sequential collection of values. A vector may be
empty. A vector may contain values of different types.
Each value in a vector is numbered starting at 0, that number is
called its index. The index is used to refer to each value when
looking them up.
Compartment-like structure
To imagine a vector, imagine a box split into some number of
equally-sized compartments. Each of those compartments has a number.
You can put a piece of data inside each compartment and always know
where to find it, as it has a number.
Note that the numbers start with 0. That may seem strange, but we
often count from zero when programming.
Syntax
Vectors are written using square brackets with any number of pieces
of data inside them, separated by spaces. Here are some examples of
vectors:
When there are a couple of turtles,
(turtle-names) command will return turtle names in the form of a
vector.
(turtle-names);=> [:trinity :neo :oracle :cypher]
Creation
The next two functions are used to make new vectors. The vector
function takes any number of items and puts them in a new vector.
conj is an interesting function that you’ll see used with all the
data structures. With vectors, it takes a vector and an item and
returns a new vector with that item added to the end of the vector.
Why the name conj? conj is short for conjoin, which means to
join or combine. This is what we’re doing: we’re joining the new
item to the vector.
Now, take a look at these four functions. count gives us a
count of the number of items in a vector. nth gives us the nth
item in the vector. Note that we start counting at 0, so in the
example, calling nth with the number 1 gives us what we’d call the
second element when we aren’t programming. first returns the first
item in the collection. rest returns all except the first item.
Try not to think about that and nth at the same time, as they can
be confusing.
Add a line: (add-turtle :neo) on the last of walk.clj file
Select this line and click “Reload Selection”
(Optional) add a turtle using REPL
Type (add-turtle :oracle) followed by enter on the bottom REPL pane
See turtle names
Type (turtle-names) on the bottom REPL pane and see the result
EXERCISE 2: Make a vector
Go to myproject’s core.clj and start InstaREPL
Make a vector of the high temperatures for the next 7 days in the
town where you live.
Then use the nth function to get the high temperature for next
Tuesday.
Maps
key value pairs
Maps hold a set of keys and values associated with them. You can
think of it like a dictionary: you look up things using a word (a
keyword) and see the definition (its value). If you’ve programmed in
another language, you might have seen something like maps–maybe
called dictionaries, hashes, or associative arrays.
Syntax
We write maps by enclosing alternating keys and values in curly braces, like so.
Maps are useful because they can hold data in a way we normally
think about it. Take our made up example, Sally Brown. A map can
hold her first name and last name, her address, her favorite food,
or anything else. It’s a simple way to collect that data and make it
easy to look up. The last example is an empty map. It is a map that
is ready to hold some things, but doesn’t have anything in it yet.
{:first"Sally":last"Brown"}{:a1:b"two"}{}
Example
When turtle received commands such that forward or right,
those return the result as a form of map of map.
assoc and dissoc are paired functions: they associate and disassociate items from a map. See how we add the last name “Brown” to the map with assoc, and then we remove it with dissoc. merge merges two maps together to make a new map.
count, every collection has this function. Why do you think the
answer is two? count is returning the number of associations.
Since map is a key-value pair, the key is used to get a value from a
map. One of the ways often used in Clojure is the examples below.
We can use a keyword like using a function in order to look
up values in a map. In the last example, we supplied the key :MISS.
This works when the key we asked for is not in the map.
Then we have keys and vals, which are pretty simple: they return
the keys and values in the map. The order is not guaranteed, so we
could have gotten (:first :last) or (:last :first).
After the creation, we want to save a new value associated to the
key. The assoc function can be used by assigning a new value to
the existing key.
Also, there’s handy function update. The function takes map and
a key with a function. The value of specified key will be the first
argument of the given function.
The update-in function works like update, but takes a vector of keys
to update at a path to a nested map.
Simple values such as numbers, keywords, and strings are not the
only types of things you can put into collections. You can also put
other collections into collections, so you can have a vector of
maps, or a list of vectors, or whatever combination fits your data.
